My kids have been great little travelers ever since they were born... some say it's because we do travel a lot... but I say, it's because I am a smart mom (like yourself) who plans ahead!!
I have my kids participate in the planning, packing and prep of the trip as much as possible. (And when they are asleep I recheck everything.) This helps build excitement.
We map our route and go online to check special interests spots along the way too.. You can photocopy the map for them to each have a copy and have them highlight the route as you go... always good to stick education in there somewhere.. hee hee.
But the trick I have always used, and is so much fun to me, is the mommy bag. No one knows whats in it, but on long trips, I take something out of it every hour. You can tailor the "stuff" that goes in the bag, but I always make sure I have
some new things.. crayons, stickers, pencils etc. I might include silly putty, mad libs, activity books, find-it books etc. depending on their age. Dollar stores are a gem for good finds cheap! I do usually let the kids pick out one new toy
that is under $5 which usually buys me the first couple hours.
I pack books on tape and fun musically tapes for kids too.
But my most favorite item in the mommy bag is the 1 minute homemade book I always pack for each kid (I have 3 also). I take several pieces of paper, fold them together in half, then get a piece of colored construction paper, fold it in half and place over the white pieces like a book cover. Then I staple the edges and write on the cover "Jack's Book, Ian's book etc."
The pictures, stories and games they create with these books is amazing.
I am also not at all uptight about keeping the van clean on a long trip. We have snacks when we are hungry, we take shoes off when they are crowding our toes and we have fun!
Hope your trip goes well. You sound like you already have a terrific attitude and a lot of spousal support... I'd say that will get you half way there. The mommy bag might help get you the rest of the way.... Good Luck!